BIOL 117 Lecture Notes - Lecture 2: Electron Shell, Linus Pauling, Covalent Bond
Lecture 2
Electron Orbitals
- electrons move around the nucleus in specific regions called orbitals
- each orbital can hold 2 electrons; 2 in the first/closest shell to the nucleus at a
maximum, and second and third shell (third and outermost shell is called the
valence shell) can hold a maximum of 8 electrons each
- an electron’s energy level (potential energy) is correlated with its distance from
the nucleus
- each electron shell has a characteristic average distance and energy level
- an electron can move from shell to shell, but only by absorbing or losing an
amount of energy equal to the difference in potential energy between its position in the old shell
and that in the new shell
i.e.: Oxygen (sub)8O =
1s = 2 ; 2s = 2, 2p(sub)x = 2, 2p(sub)y = 1, 2p(sub)z = 1
Carbon (sub)6C =
1s = 2 ; 2s = 2, 2p(sub)x = 1, 2p(sub)y = 1, 2p(sub)z = 0
Nitrogen (sub)7N =
1s = 2 ; 2s = 2, 2p(sub)x = 1, 2p(sub)y = 1, 2p(sub)z = 1
-Chemical properties of an atom depends on the number of valence electrons:
-an atom with a complete valence shell is unreactive or inert
-an atom with an incomplete valence shell is chemically reactive (tends to form chemical
bonds until it has two (H) or either electrons N, O, C to fill the valence shell)
-atoms with the same number of valence electrons show similar chemical behavior
out of NOCH, Oxygen has the most valence electrons
-The formation and function of molecules depend on chemical bonding between atoms:
1 . Covalent bonds - formed by sharing a pair of valence electrons
a. H2 (hydrogen) - 2 hydrogen atoms can form a single bond
b. OX (oxygen) - 2 oxygen atoms share 2 pairs of electrons to form a double bond
c. H2O (water) - 2 hydrogen atoms and 1 oxygen atom are joined by covalent bonds to
produce a molecule of water
d. CH4 (methane) - 4 hydrogen atoms can satisfy the valence of 1 carbon atom, forming
methane
-nonpolar covalent - formed by equal sharing of electrons between atoms
-polar covalent - formed by unequal sharing of electrons between atoms
-the determinant of whether youre going to get a polar or nonpolar covalent bond is the
ELECTRONEGATIVITY (an atoms ability to hold and attract electrons) **the more filled
the outer electron shell is, the higher the electronegativity** **the fewer electron shells,
the higher electronegativity as well**
-scale determined by Linus Pauling (of electronegativity) = tells if polar or nonpolar even if
its an ionic bond
-oxygen = 3.5 (high)
-nitrogen = 3.0
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Document Summary
Electrons move around the nucleus in speci c regions called orbitals. An electron"s energy level (potential energy) is correlated with its distance from the nucleus. Each electron shell has a characteristic average distance and energy level. Chemical properties of an atom depends on the number of valence electrons: An atom with a complete valence shell is unreactive or inert. An atom with an incomplete valence shell is chemically reactive (tends to form chemical bonds until it has two (h) or either electrons n, o, c to ll the valence shell) Atoms with the same number of valence electrons show similar chemical behavior out of noch, oxygen has the most valence electrons. The formation and function of molecules depend on chemical bonding between atoms: Nonpolar covalent - formed by equal sharing of electrons between atoms. Polar covalent - formed by unequal sharing of electrons between atoms.
